Corrosion has always and is presently a significant problem in the refining and petrochemical industries by reason of equipment replacement costs and system downtime associated therewith.
For example, in refinely operation, the crude oil itself commonly contains corrosive impurities such as salts, sulfur compounds, naphthenic and other organic acids such as acetic and propionic, nitrogen compounds and inorganic acids such as hydrochloric acid. Corrosion by crude oil impurities is accelerated by the high temperatures (e.g., 100.degree. F.-1000.degree. F.) commonly encountered when oils or petrochemicals are processed. In many cases, crudes are processed at temperatures within the range of 600-1000.degree. F.
Furthermore, upon heating or subjecting the crude oil to various catalytic processes, the impurities are converted more completely to volatile and water-soluble compounds. Examples of these are: HCl; CO; CO.sub.2;, formic, acetic and other volatile carboxylic acids; H.sub.2 S; SO.sub.2 ; SO.sub.3 and NH.sub.3. Besides NH.sub.3, other basic compounds, especially monoethanolamine, diethanolamine, etc. can be present because of the rerouting of various refinery "slop" streams for reprocessing. In catalytic units, more NH.sub.3 and H.sub.2 S are formed relative to the stronger inorganic and organic acids than in simple distillation equipment. Thus the composition and pH of the corrosive aqueous phase can vary substantially and be dependent upon crude oil contaminants and processing. Furthermore, the pH within a given system will change as water condensation proceeds due to the different distribution function of the volatile species among hydrocarbon, aqueous and vapor phases.
In FCC units and certain crude units, aqueous condensates can be alkaline in nature Data for many conventional corrosion inhibitors show that significantly high treatment levels are required for corrosion protection of ferrous and non-ferrous containers in high pH, sulfidic environments The use of such high treatment levels is economically unattractive.
This is especially true for admiralty brass and would be expected to be true for other copper alloys (e.g., cupronickels). This phenomenon is a result of the stability of copper/ammonia (or amine) complexes. At high pH (above 7.0), the amines convert from cationic to neutral forms with a corresponding increased propensity to complex copper ions in solution This accelerates corrosion by removal of the metal atoms and corrosion products
The present invention has particular utility in overhead condensing systems of various refinery and petrochemical processing units. Usually the units themselves and their associated piping are of sufficient thickness to provide many years of service under general corrosive attack. In these units, however, tower trays and heat exchanger tubes are relatively thin due to weight constraints or to allow high rates of heat transfer. Thus, these components are subject to shorter service life, especially if localized corrosion, such as underdeposit corrosion, occurs. This is more likely to occur in alkaline systems where various ammonia or amine salts can form deposits leading to localized attack. The tubes are usually made of mild steel due to the low cost of this metal, but admiralty brass and cupronickel are frequently used. In extremely aggressive systems titanium heat exchanger bundles have been used.
Besides corrosion inhibition, another desirable property of any inhibitor is substantial solubility in the hydrocarbon fluids being treated. Lack of sufficient solubility can lead to deposition or plugging problems. The inhibitors are usually fed as concentrated solutions (percent levels) into a stream which is primarily vapor and of high temperature. The inhibitor must stay soluble under these conditions in order to contact and film the metal parts and to prevent agglomeration and deposition. This is particularly important in catalytic units which produce light hydrocarbon products. As discussed above, the formation of deposits on thin parts such as exchanger tubes can lead to frequent leaks and downtime due to underdeposit corrosion or physical blockage A further effect of such deposits is to impede heat transfer. Since most corrosion inhibitors contain amine functionality, they can be aggressive to copper alloys. Also, because these inhibitors partition to the oil phase in preference to water, they are returned to the tower via reflux streams. As the reflux vaporizes, insufficient solubility of the filmer will result in deposition on tower trays. Accumulation of such deposits will interfere with establishment of vapor-liquid equilibria within the tower and cause excessive pressure drops within the tower.
The present invention combines both excellent corrosion inhibition and increased hydrocarbon solubility properties.